How does ViewModel cache LiveData?

Question

I saw all of the following scenarios in different example projects from Google's Codelabs and other sources and do not fully understand where the values from the LiveData object are retrieved from.

Scenario 1 - Current Understanding:

According to https://developer.android.com/.../viewmodel one reason to use a ViewModel is to store/cache UI related data that I want to re-use after the corresponding UI has been rebuild after a configuration change.

Given the following simplified ViewModel and Repository: After updateName() is called the first time, the LiveData object of _currentName contains a String. If the UI is then rebuild after a screen rotation, the view that needs to display the current name requests it by observing currentName which in turn returns the value of the LiveData object that is contained in the field of the _currentName property. Am I correct?

ViewModel

class NamesViewModel(): ViewModel() {
    
    private val respository = NamesRepository()

    private val _currentName: MutableLivedata<String?> = MutableLiveData(null)
    val currentName: LiveData<String?> get() = this._currentName

    ...

    // Called as UI event listener.
    fun updateName() {
        this._currentName.value = this.repository.updateName()
    }
}

Repository

class NamesRepository() {
    
    fun updateName(): String {
        val nextName: String     
   
        ...

        return nextName
    }
}

Scenario 2:

What happens if the UI is rebuild after a screen rotation in the following case? _currentName in the ViewModel 'observes' currentName in the repository, but it still is a property and therefore stores its own LiveData object in its field. When the view then requests currentName from the ViewModel, the value is retrieved from the LiveData object that is contained in the field of the _currentName property in the ViewModel. Is this correct?

ViewModel

class NamesViewModel(): ViewModel() {
    
    private val respository = NamesRepository()

    private val _currentName: LiveData<String?> = this.repository.currentName
    val currentName: LiveData<String?> get() = this._currentName

    ...

    // Called as UI event listener.
    fun updateName() {
        this.repository.updateName()
    }
}

Repository

class NamesRepository() {

    private val _currentName: MutableLivedata<String?> = MutableLiveData(null)
    val currentName: LiveData<String?> get() = this._currentName
    
    fun updateName() {
        val nextName: String     
   
        ...

        this._currentName.value = nextName
    }
}

Scenario 3:

In the following scenario, if the UI is rebuild and a view requests currentNam from the ViewModel, where is the requested value stored? My current understanding is, that currentName falls back to the field of the property _currentName in the repository. Isn't that against the idea of the ViewModel to store relevant UI data to be re-used after a configuration change? In the case below, it might be no problem to retrieve the value from the repository instead of the viewModel, but what if the repository itself retrieves the value directly from a LiveData object that comes from a Room database? Wouldn't a database access take place every time a view requests _currentName from the viewModel?

I hope somebody can clarify the situation more, in order to understand how to cache UI related data in the viewModel the correct way (or at least to understand what are the incorrect ways).

ViewModel

class NamesViewModel(): ViewModel() {
    
    private val respository = NamesRepository()

    val currentName: LiveData<String?> get() = this.repository.currentName

    ...

    // Called as UI event listener.
    fun updateName() {
        this.repository.updateName()
    }
}

Repository

class NamesRepository() {

    private val _currentName: MutableLivedata<String?> = MutableLiveData(null)
    val currentName: LiveData<String?> get() = this._currentName
    
    fun updateName() {
        val nextName: String     
   
        ...

        this._currentName.value = nextName
    }
}

Answer 1

To answer your question scenario#1 is correct usage of LiveData.

Firstly, LiveData is not responsible for caching, it is just LifeCycleAware Observable, given that caching is done at ViewModel, when your activity recreates due to any configuration changes, android will try to retrieve the existing instance of ViewModel, if found then it's state and data are retained as is else it will create a new instance of ViewModel.

Second, using LiveData in repository is a bad idea at many levels, repository instances are held by ViewModel and LiveData are part of Android Framework which makes repositories rely on Android Framework thus creating problems in Unit Testing. Always use LiveData only in ViewModels.